Moderate hyperhomocysteinemia and endothelial dysfunction are consistent findings in uremic patients. Although an exceedingly high incidence of cardiovascular disease and stroke has been shown in dialysis patients, several traditional risk factors are relatively limited predictors. Hyperhomocysteinemia could be a principal candidate for endothelial dysfunction. Recent findings suggest that hyperhomocysteinemia may impair endothelial function by the generation of oxygen species and decreased nitric oxide (NO) bioavailability. However, the precise mechanisms underlying the link between hyperhomocysteinemia and impaired endothelial function in chronic renal failure remain unclear. Endothelial function was evaluated by the response to reactive hyperemia and donor of NO. We observed impairment in both endothelium-dependent and endothelium-independent vasodilation in dialysis patients. These data suggest that patients with chronic renal failure may have defective NO-mediated function in the endothelium and smooth muscle of vessels. Most reports have shown only impairment of endothelium-dependent vasodilation, whereas another report observed impaired smooth muscle function and intact endothelial function. Only a few previous observations included a full set of vascular function data, such as baseline vessel diameter, reactive hyperemia, and responses of endothelium to hyperemia and NO donor, although all these observations could be essential for comparison with other reports. Treatment with folic acid was reported to reduce plasma homocysteine levels, but not to normal levels, and failed to improve impaired endothelial function in patients in a predialysis phase and on maintenance dialysis therapy. Another investigation, directed at reducing homocysteine levels in earlier stages of renal failure, may be necessary to clarify the link between hyperhomocysteinemia and endothelial function.